Tidy up favourite scanning

Transmit no longer stores favourites in the prefs file (where the plugin was previously looking), but in an sqlite file.
The easiest way of accessing favs is now with SB/AS (I couldn't be bothered sifting through the SQLite file), but this has the penalty of having to launch Transmit, so its effects have been kept to a minimum (only scan when absolutely necessary)

Note: you may think the test in `indexIsValid...` would fail the first time you enable the catalog preset in the prefs, but it seems QS force scans the first time, so we're safe.

Plus, I think it's acceptable in this case to launch Transmit/only scan when transmit is running since:

* If you → into Transmit.app then you're wanting to open a favourite. Hence you need Transmit running in the end anyway
* You can only create favourites when Transmit is running, so QS will only rescan in that window.